SAN PEDRO SULA, Honduras — A new caravan of migrants is forming in Honduras, and even ahead of its scheduled departure at dawn on Tuesday, battle lines were being drawn to the north, with some vowing to help them on their journey north, and others to block them.

For President Trump, the timing of the caravan offered fresh ammunition in his fight with Congress over the $5.7 billion he wants for an enhanced border wall between Mexico and the United States. The dispute has led to a partial shutdown of the federal government.

As he did last fall, when another caravan made the same trek, Mr. Trump portrayed the migrants — who say they are trying to escape poverty and violence, and who in seeking asylum are exercising a legal right — in an ominous light.

“There is another major caravan forming right now in Honduras, and so far we’re trying to break it up, but so far it’s bigger than anything we’ve seen,” Mr. Trump said on Thursday. “And a drone isn’t going to stop it and a sensor isn’t going to stop it, but you know what’s going to stop it in its tracks? A nice, powerful wall.”

Despite Mr. Trump’s assertions, nobody knows how many people will leave on Tuesday and how many more may join the walkers as they cross Guatemala, reach southern Mexico and make their way to the United States border.

It was also unclear on Sunday who put the plan in motion for this caravan.

Héctor Romero, 37, has decided he is going to join the caravan on Tuesday. “I have had only two days’ work a week for the past three months and that barely covers expenses,” said Mr. Romero, who collects bus fares in a small town about 40 miles west of San Pedro Sula, the city from which the caravan intends to start. “I didn’t have the courage to go last time, but this time I do.”Tijuana has been struggling to accommodate an influx of migrants from Central America.CreditMeghan Dhaliwal for The New York Times

Tijuana has been struggling to accommodate an influx of migrants from Central America.CreditMeghan Dhaliwal for The New York Times

The divorced father of four is taking his 12-year-old daughter with him, believing that it may improve his chances with United States immigration authorities.

The first challenge to the migrants may come from their own governments. The deeply unpopular presidents of Honduras and Guatemala, both tarnished by scandal, are eager to maintain the support of the Trump administration. Halting the caravan could help them do that.

On Thursday, the chargé d’affaires in the American Embassy, Heide B. Fulton, traveled to the border with Guatemala to tape a plea to migrants. “Don’t let yourself be fooled,” she said. “Don’t invest your time and money in a journey that is destined to fail.”

In Mexico, the new government, led by the leftist President Andrés Manuel López Obrador, which took office on Dec. 1, says it will deal with the migrants more humanely than the preceding administration.

Officials say they want to avoid a repetition of the “horror” earlier migrants endured as they tried to avoid detection — and deportation — on the perilous trek across Mexico.

“Our vision is that migrants are not criminals, much less do they constitute a threat to the security of Mexico or the United States,” Mexico’s interior minister, Olga Sánchez Cordero, said last week in a speech to Mexican diplomats, promising an end to massive deportations.The administration of Andrés Manuel López Obrador, Mexico’s new president, has said it will deal with the migrants more humanely than the preceding administration.CreditMeridith Kohut for The New York Times

The administration of Andrés Manuel López Obrador, Mexico’s new president, has said it will deal with the migrants more humanely than the preceding administration.CreditMeridith Kohut for The New York Times

More than 300,000 Central Americans entered Mexico last year, most of them illegally, and an estimated 80 percent of them were bound for the United States border, Ms. Sánchez Cordero said.

She said migrants in a new caravan who enter the country at official crossing points and register would be granted visas to stay and work in Mexico or permits to travel under the supervision of migration authorities toward the United States border. But those who cross into Mexico illegally, she said, will be deported.

The government’s new policies will be put to the test when the caravan arrives, said Gustavo Mohar, a former migration official in Mexico’s Interior Ministry. “You cannot resolve the problem,” he said. “You manage it intelligently, cautiously, realistically.”

He said success, particularly with so much international attention focused on the Central American migrants, would give Mexico “moral authority with the United States.”

Traveling in caravans offers safety in numbers from the criminal groups and corrupt officials who prey on migrants, activists say. But the size of recent caravans has become “uncontrollable,” said Irineo Mujica, a member of Pueblo Sin Fronteras, a transnational group that accompanied earlier caravans in Mexico. It is not organizing this new one.Many migrants who made it to Tijuana are stuck there. Now, more may be on the way.CreditMeghan Dhaliwal for The New York Times

Many migrants who made it to Tijuana are stuck there. Now, more may be on the way.CreditMeghan Dhaliwal for The New York Times

As the new caravan prepares to leave, the experience of the last one seems to be guiding the response of governments and people along the way.

That previous exodus was a tale told through images: migrants wading across the Suchiate River, which marks the border between Guatemala and Mexico; masses of people filling country roads and cramming into pickup trucks; and the central squares of provincial Mexican cities transformed into cluttered campsites.

When the caravan — almost 6,000 strong — reached Tijuana, the migrants found that a high fence and a very long wait to ask for asylum still separated them from the United States. Migrant shelters overflowed and conditions in them quickly worsened. Some migrants gave up.

But back home in Honduras, the trials of previous caravans have not been a deterrent for those considering joining the new one. The danger and frustrations pale beside the overwhelming fear of being sent back home, said Sister Lidia de Suazo, the coordinator of pastoral care for migrants at the Roman Catholic archdiocese in the Honduran capital, Tegucigalpa.

“The majority of those who went with the October caravan were not deported,” she said. “So that sends the message back to the countries of origin, and people say, ‘Let’s go too because they won’t deport us.’ ”

Miroslava Cerpas, the migration coordinator at the Center for Research and Promotion of Human Rights in Tegucigalpa, said: “We explain the dangers to them. It doesn’t matter what might happen to them, they are going to leave. Many don’t know where they are going but they know what they are leaving behind.”

Some of the migrants in Tijuana were considering going south to join the new caravan and accompany those making the trip for the first time. Omar Rivera, 39, a construction worker from El Salvador, was one of them.

“A lot of people are coming,” he said as he prepared to board a bus heading south, “and they need our help.”

Jeff Ernst reported from San Pedro Sula, Honduras, Elisabeth Malkin from Mexico City and Paulina Villegas from Tijuana, Mexico.